Luminescent organic materials have attracted significant recent attention due to their role in a variety of functional devices. One particular area of interest comprises organic light emitting diodes (OLEDs), which operate based on charge injection and radiative recombination in multilayer devices. A related but more specialized application that is based on similar principles of charge transport and exciton mobility involves organic materials for the detection of ionizing radiation. This application serves as the basis for nuclear non-proliferation detection of illicit nuclear materials such as highly enriched uranium or plutonium.
Organic scintillators possess the unique ability to discriminate ionization caused by fast neutron recoils on nuclei from that caused by Compton scattering of gamma-rays on electrons, owing to differences in the emission kinetics of the produced light pulses. These differences are evident in the relative fraction of light produced via prompt singlet fluorescence versus delayed triplet-triplet annihilation (TTA). In practice, nuclear recoils from fast neutron interactions produce a greater proportion of delayed luminescence than gamma-rays and can be identified by their characteristic pulse shape. This is due to a phenomenon known as ionization quenching, which leads to a reduction in the relative proportion of prompt fluorescence for neutron versus gamma-ray events. In mixed fluorophore systems, such as plastics and liquids, Förster resonant energy transfer also plays a role. This technique for identifying the type of incident particle is known as pulse-shape discrimination (PSD).
While this pulse-shape discrimination technique is effective in some materials such as trans-stilbene single crystals or liquid scintillation mixtures, there are several limitations that preclude their use in critical applications such as radiation portal monitors used at border crossings and ports-of-entry. First, PSD is easily disrupted by the presence of disorder or impurities. This is primarily due to a reliance upon TTA to provide the delayed emission component. TTA is a bimolecular recombination process that requires Dexter electronic interaction between two triplet excited states, the probability of which decreases exponentially as a function of distance. The presence of disorder or impurities decreases the effective triplet exciton lifetime due to a higher density of trapping sites that compete with TTA. Several limitations and challenges remain in maximizing the various properties of these types of materials.
The combination of these limitations has led to significant interest in non-crystalline organic scintillators based on polymers and organic liquids. See Knoll, G. F. Radiation Detection and Measurement, 4th ed.; John Wiley & Sons: Hoboken, N.J., 2010. Various strategies have been employed to achieve neutron/gamma PSD in these materials, although the obtained scintillation light yields and discrimination performance have been found to be distinctly inferior to single crystals. See Bourne, M. M.; Clarke, S. D.; Adamowicz, N.; Pozzi, S. A.; Zaitseva, N.; Carman, L. Nucl. Instrum. Methods Phys. Res., Sect. A 2016, 806, 348-355 and Feng. P. L.; Villone, J.; Hattar, K.; Mrowka, S.; Wong, B. M.; Allendorf, M. D.; Doty, F. P. IEEE Trans. Nucl. Sci. 2012, 59, 3312-3319.
The performance of an organic scintillator is influenced by the efficiency and kinetics of radiative decay processes that are associated with ion recombination and exciton transport. It is difficult to maximize scintillation performance in organic scintillators while also achieving stability in optical, electronic, and morphological properties, and doing so with cost-effective materials.